We have a slackline that is shared with my sisters family and pretty much stays at her house, actually DDS gym bought one for camp a few years ago, but the kids couldn't attempt it on their own of course. DD loves it, over the summer when visits relatives she got some decent practice and came close to making it across. I have brothers that are a bit extreme (slack lining, base jumping, baselining, and rock climbing) so being around them brings out her adventurous side.
I went to an accrobranch centre with my kids this summer and was forced to walk along a 20 metre slackline around 10 metres above the ground (with safety harness, of course) - it was flipping terrifying. As was skateboarding along a wire, scooting across a wire and sledding across one. Are the French the only crazy ones or do other countries have this form of torture-for-parents, with jolly advertisments stating that 'everyone can join the fun', which is when you start to wish you hadn't taught your kids to read?